Modern fans systems used in commercial structures that employ variable air volume duct distribution systems and use fan speed controls to reduce the duct pressure and airflow during low load periods are highly efficient but have a limited range of operation. The minimum flow is usually determined by the surge effect of the fans. This effect can reduce the range of operation and potentially reduce the power savings available at reduced flow.
With the increasing popularity of fan array systems that employ two or more fans operating in parallel, it is possible to extend the minimum range of a fan array system by switching off one or more of the fans in an array as they are about to go into surge thereby allowing the fans that remain operating to move away from their surge point and supply lower flow than would be possible otherwise. This invention addresses how to control an array of fans to avoid the surge effect and enable the array of fans to operate at lower flows and lower power levels than would be otherwise possible.
FIG. 1 is a schematic illustration of an air handling system of the type commonly referred to as a “variable air volume” system or a “VAV” system designated generally by the reference numeral 100. Most modern air conditioning systems on large commercial systems employ large air handlers to control the inside air conditions in multiple zones Z1-Z4 or rooms in the structure. Because the heating and or cooling loads in each of these zones are independent and variable it is desirable to provide individual control in each zone. A large air handler 102 is connected to a supply air distribution system of ductwork designated generally at 104.
Variable air volume boxes 106 are installed in the ductwork to control the airflow to a plurality of zones in the building served by the fans 108 in the air handler 102. Each of the variable air volume boxes 106 has an air control damper built into it. The damper is used to throttle the air flow going to the zone. In this way, one air handler can serve several zones in the building and maintain good temperature control in each of those zones.
In this type of system, a duct pressure controller 114 senses supply duct pressure and controls the fan speed accordingly. The pressure controller 114 sends control signals to the fan speed control 116 to maintain a set pressure in the duct. The speed controller then changes the fan running speed in order to accomplish this. If the majority of the variable air volume boxes 106 open up, the pressure in the duct will fall, and the duct pressure control system will increase the speed of the fans 108 to compensate. Likewise, if the majority of the boxes close up, the duct pressure will tend to rise and the duct pressure control system will slow the fan speed. The pressure in the supply duct will then be constant and allow the dampers in the variable air volume boxes 106 to operate in a relative stable manner. This control pressure, in effect, sets a minimum distribution system pressure loss.
The distribution system 104 is only part of the pressure loss that the fan 108 must overcome. Cooling coils 120, heating coils 122, and filters 124, as well as dampers, louvers, and other items not shown, provide resistance to air flow in the form of pressure loss. These losses typically vary with the square of the air flow through those devices. At design (maximum) flow, these other losses typically are a large part of the pressure loss the fans 108 must overcome. Because these losses are falling off with the square of the flow, they reduce rapidly as the flow is reduced. For example, at twenty-five percent (25%) of design flow, these losses are only six and one-fourth percent (6.25%) of their design value, and they become a small part of the pressure loss that the fans 108 must overcome. FIG. 2 shows a typical flow resistance curve in a modern VAV system.
Centrifugal fans are commonly used in air handling systems because they are highly efficient, compact, and are easy to apply. As the system flow resistance is increased on a centrifugal fan, the flow will decrease and the pressure will rise. At a certain point of pressure and flow, the fan will go into surge. Surge is a well known condition in fans; it is the point where the fan blades experience flow separation similar to the phenomena known as stall on an aircraft wing. The combination of pressure and flow at which a fan goes into surge is a function of the operating speed and can be determined easily by test. Fan manufacturers publish data that defines when a fan will go into surge. This data can be defined in the form of a constant, Ks, for use in the following equation:SPsurge=Ks×CFM2 
When the pressure is higher than SPsurge for any given flow, the fan is in surge. When a fan goes into surge, it produces pressure pulsations that can cause duct rumble, noise, and excessive vibration. If the fan(s) operate in surge, these pulsations can cause damage to the duct distribution system, the air handler, and the fan(s). It is important to avoid fan operation in surge. A plot of the fan surge pressure (SPsurge) verses fan airflow is known as the surge curve.
FIG. 3 shows the fan pressure delivery verses flow curves for three different fan speeds. The maximum speed is typically the design speed. This design speed is selected so that the fan(s) deliver the desired airflow at the expected system pressure. This is represented by the intersection of the system curve and the fan curve at design speed. This is typically the maximum flow of the fan(s). When less flow is required to meet the heating or cooling load in the building, the speed of the fan(s) is reduced. Reducing the fan speed changes the fan curve according to a relationship known as the Fan Laws, which state for any point on a fan curve:Airflow2=Airflow1×(Speed2/Speed1)Pressure2=Pressure1×(Speed2/Speed1)2 
This is illustrated by FIG. 3 where the three fan curves are labeled “Design Speed,” “Reduced Speed,” and “Min Speed.” These are shown to represent three distinct operating conditions to illustrate the effect of lowering fan speed. The fan speed is infinitely variable between the maximum and the minimum.
Because of the need to maintain the control pressure, the fan(s) on a VAV system will go into surge when the flow is low enough. The flow at which the fan(s) go into surge is at the point where the VAV system's flow resistance curve intersects the fan surge curve. At lower flows the pressure will be higher than the surge pressure, and the fan(s) will be in surge. FIG. 3 also illustrates the point where fan(s) go into surge by plotting the surge curve on the same chart as the system resistance curve. This flow is commonly known as the surge point.
The surge point defines the minimum flow at which a VAV system can safely operate. The minimum flow of a given system is a function of the selection of fan size and the control pressure. Usually several different size fans can be selected to meet the flow and pressure requirements of a system. There will be an optimum size that results in the most efficient fan selection. If smaller fans are selected, they will run at higher speed and lower efficiency than the optimum selection. A larger (and more efficient) fan will have a higher flow at the surge point and will therefore have a more limited range of operation.
A higher control pressure will also result in a higher flow at the surge point for any fan selection. A typical VAV system will need to operate at flows as low as thirty to forty percent (30%-40%) of the design (maximum) flow. Often times the most efficient fan selection will result in a fan(s) that will only turn down to fifty to sixty percent (50%-60%) of design flow, which is not enough to satisfy the needed operating range of the building.
In order to decrease the flow at the surge point and increase the operating range of the system, designers often choose smaller and less efficient fans. While this accomplishes the goal of increasing the operating range of the system, it has other undesirable effects. Because the fans are less efficient, they consume more power. This increases energy consumption, peak electrical demand, and often the installed motor horsepower. This results in higher cost of installation and higher operating costs. When fans need to deliver less than thirty to forty percent (30%-40%) of design (maximum) flow, designers often add a flow bypass device to divert air flow around the fan(s) to accomplish this high turndown. This flow bypass substantially reduces the efficiency of the fan(s) when operating at low loads.
Fan arrays are becoming increasingly more popular in large commercial air handlers. Many of these air handlers are used on variable air volume (VAV) systems. A fan array is two or more fans operating in parallel to meet the needs of the air handling system. Often six or more fans are used in a fan array. Because of this plurality of fans, it is possible to increase the operating range of the fan array system by shutting off fans when the flow is reduced to the point where the fan array would go into surge. By shutting off a fan when the array is at or near the surge point, the remaining fans will be sped up by the duct pressure controller to maintain the required system flow. This will result in increased flow per fan, which drives those fans away from their surge point. If the building flow requirement continues to fall, additional fans can be shut off to the extent they are available to allow further flow reductions.